Computers include general purpose central processing units (CPUs) that are designed to execute a specific set of system instructions. A group of processors that have similar architecture or design specifications may be considered to be members of the same processor family. Examples of current processor families include the Motorola 680X0 processor family, manufactured by Motorola, Inc. of Phoenix, Ariz.; the Intel 80X86 processor family, manufactured by Intel Corporation of Sunnyvale, Calif.; and the PowerPC processor family, which is manufactured by Motorola, Inc. and used in computers manufactured by Apple Computer, Inc. of Cupertino, Calif. Although a group of processors may be in the same family because of their similar architecture and design considerations, processors may vary widely within a family according to their clock speed and other performance parameters.
Each family of microprocessors executes instructions that are unique to the processor family. The collective set of instructions that a processor or family of processors can execute is known as the processor's instruction set. As an example, the instruction set used by the Intel 80X86 processor family is incompatible with the instruction set used by the PowerPC processor family. The Intel 80X86 instruction set is based on the Complex Instruction Set Computer (CISC) format. The Motorola PowerPC instruction set is based on the Reduced Instruction Set Computer (RISC) format. CISC processors use a large number of instructions, some of which can perform rather complicated functions, but which require generally many clock cycles to execute. RISC processors use a smaller number of available instructions to perform a simpler set of functions that are executed at a much higher rate.
The uniqueness of the processor family among computer systems also typically results in incompatibility among the other elements of hardware architecture of the computer systems. A computer system manufactured with a processor from the Intel 80X86 processor family will have a hardware architecture that is different from the hardware architecture of a computer system manufactured with a processor from the PowerPC processor family. Because of the uniqueness of the processor instruction set and a computer system's hardware architecture, application software programs are typically written to run on a particular computer system running a particular operating system.
Computer manufacturers want to maximize their market share by having more rather than fewer applications run on the microprocessor family associated with the computer manufacturers' product line. To expand the number of operating systems and application programs that can run on a computer system, a field of technology has developed in which a given computer having one type of CPU, called a host, will include an emulator program that allows the host computer to emulate the instructions of an unrelated type of CPU, called a guest. Thus, the host computer will execute an application that will cause one or more host instructions to be called in response to a given guest instruction. Thus the host computer can both run software design for its own hardware architecture and software written for computers having an unrelated hardware architecture. As a more specific example, a computer system manufactured by Apple Computer, for example, may run operating systems and program written for PC-based computer systems. It may also be possible to use an emulator program to operate concurrently on a single CPU multiple incompatible operating systems. In this arrangement, although each operating system is incompatible with the other, an emulator program can host one of the two operating systems, allowing the otherwise incompatible operating systems to run concurrently on the same computer system.
When a guest computer system is emulated on a host computer system, the guest computer system is said to be a “virtual machine” as the guest computer system only exists in the host computer system as a pure software representation of the operation of one specific hardware architecture. The terms emulator, virtual machine, and processor emulation are sometimes used interchangeably to denote the ability to mimic or emulate the hardware architecture of an entire computer system. As an example, the Virtual PC software created by Connectix Corporation of San Mateo, Calif. emulates an entire computer that includes an Intel 80X86 Pentium processor and various motherboard components and cards. The operation of these components is emulated in the virtual machine that is being run on the host machine. An emulator program executing on the operating system software and hardware architecture of the host computer, such as a computer system having a PowerPC processor, mimics the operation of the entire guest computer system.
The emulator program acts as the interchange between the hardware architecture of the host machine and the instructions transmitted by the software running within the emulated environment. This emulator program may be a host operating system (HOS), which is an operating system running directly on the physical computer hardware. Alternately, the emulated environment might also be a virtual machine monitor (VMM) which is a software layer that runs directly above the hardware and which virtualizes all the resources of the machine by exposing interfaces that are the same as the hardware the VMM is virtualizing (which enables the VMM to go unnoticed by operating system layers running above it). A host operating system and a VMM may run side-by-side on the same physical hardware.
A virtual device is a logical device, implemented in software, which corresponds to some kind of actual or idealized physical device. Generally, there are two approaches for modeling virtual devices: the “hardware virtual device” approach, which directly models an existing piece of hardware, and the “idealized virtual device” approach, which is not a mere reflection of the physical hardware, but is optimized for the virtual machine (VM) environment.
The hardware virtual device approach offers advantages in regard to compatibility; as the virtual device acts just like a real device in every respect, software that has been designed to interact with that device (e.g., a driver) will work with a hardware virtual device without modification. However, hardware virtual devices are at a disadvantage when it comes to performance, as physical hardware is often difficult to emulate with a virtual device without its incurring significant overhead costs (and inefficiencies) because hardware designers generally do not take into consideration virtualization issues and, thus, hardware virtual devices are often noticeably slower than their real hardware counterparts.
Idealized virtual devices, on the other hand, provide significant freedom for developers to design a virtual device that is both easy to implement and efficient to use. Because the design of an idealized virtual device does not need to conform to the limitations imposed by the physical hardware design, idealized virtual devices can be optimized for use within a VM environment. Furthermore, developers of idealized virtual devices do not need to concern themselves with the subtle side effects (such as, for example, timing and state changes) that existing software might rely on for correct operation. Moreover, developers can also create idealized virtual devices that are analogous to hardware that does not, in fact, exist—for example, a virtual device that allows for communication between a guest system and a host system. However, there is a risk that compatibility issues may arise when the idealized virtual device approach is used, as the virtual device may not, in fact, operate exactly like the real device in every respect, and software that has been designed to interact with that physical device (e.g., a driver) may not work correctly or at all with an idealized virtual device without modification.
Presently, the set of virtual devices (idealized or hardware) available in a virtual machine are predetermined at the time when the virtual machine software is written, and current virtual machine products do not include a means for adding new virtual devices. This means that the number and variety of devices is limited to those that are included in the virtual machine software. As technology advances and the number of devices (physical and virtual) continues to escalate, the ability to be able to add new virtual devices to virtual machine software has become more critical. What is needed is a way to dynamically add new virtual devices, as needed, in a virtual machine environment.
In addition to the needs of the virtual machine environment, there is also a general need for a means by which new virtual devices may be added to a system to support both the development of hardware as well as the develop of software drivers for that hardware. In regard to new hardware, there is a need for quality testing to find and eliminate bugs other unanticipated errors in a new hardware device. Generally the approach to this type of testing is wait until after the hardware devices have been designed and a prototype has been manufactured. However, this design-prototype-test process is inefficient in that, if errors are found during testing, the process starts again from the beginning and thus results in long development cycles for new devices. What is needed is a way to decrease the time to market for physical device products. Similarly, conducting complete testing of new device driver software for physical hardware devices, both old and new, is time consuming at best and, in some cases, difficult or even impossible to complete because inducing a physical hardware device to produce a wide variety of errors for handling by the device driver in a timely and meaningful fashion is difficult. The result is that many of the paths of driver code in the software driver are not fully exercised because the physical hardware devices upon which the tests are run do not (and really cannot) exhibit the full range of errors during a finite testing period. Therefore, what is needed is a way to reduce the time needed for, and to improve the quality of, device driver software testing.
Virtual devices seemingly offer a solution in both regards: a hardware virtual device mirroring the capabilities of a proposed physical hardware device could be used to test certain hardware capabilities and compatibilities before a physical prototype is developed, thereby shortcutting the design process as it is easier to modify virtual device than it is to modify a physical prototype device. Similarly, for testing new device driver software, an idealized virtual device could be used to raise all of the desired hardware device errors necessary to fully test the new device driver (where the virtual device is “idealized” in the sense of creating the desired errors, thereby eliminating the need to physically create a hardware device to purposely do so). However, the shortcoming to this approach is the fact that virtual devices tend to be hardcoded-that is, they tend to be written directly into the monolithic container of the virtual computing environment, and thus they are not easily modifiable. For example, in existing virtual machine environments, the virtual machine loads only those virtual devices that it is hardcoded to load. Therefore, what is needed in the art is a virtual computing environment that allows dynamic and modifiable loading of new virtual devices.